Wikipedia
PubSub was a prospective search engine for searching blogs, press releases, Usenet, USGS earthquake alerts, SEC filings and FAA Flight Delay information.
The site, founded in 2002 by Bob Wyman and Salim Ismail, operated by storing a user's search term, making it a subscription, and checking it against posts on blogs which ping the search engine. When a new match was found, the user was notified, even if it occurred months after the initial search. This feature led PubSub to call itself a matching engine.
Results could be read on the service's website, or on an optional sidebar, available for both Internet Explorer and Mozilla Firefox, written by Malcolm Pollack and Duncan A. Werner respectively, or via RSS. Results could also be delivered to remote systems via Restful APIs, email or XMPP